We have developed a cell culture method which can distinguish between cultured skin fibroblasts from normal individuals and cystic fibrosis (CF) patients. This method relies upon the demonstration that CF fibroblasts are significantly more resistant, as measured by cell survival, than normal cells to the cytotoxic effects of ouabain, steroid hormones, or cAMP. This drug resistant phenotype has been observed in all of 13 CF strains examined when compared to any of 10 different normal control strains. Dexamethasone, a synthetic glucocorticoid, has been the cytotoxic agent used most frequently in studies of fibroblasts from affected patients and their obligate heterozygous parents. The results of these studies showed that the dexamethasone survival assay could reliably distinguish between homozygous CF, heterozygous CF, and normal fibroblasts. From the results of preliminary work, it appears that the assay may also be able to detect a CF fetus based on studies of cells grown from amniotic fluid specimens. The object of this proposal is to allow us to investigate further the possible clinical applications of our work on drug resistance in CF cells including studies of (a) lymphocytes; (b) amniotic fluid cells; and (c) methods of further developing and improving the fibroblast assay system in order to further test its reliability and clinical usefulness.